https://www.allaboutdogfood.co.uk This will keep you busy.
Personally I go for grain free. Also I wouldn't buy at the vets, they charge more, and promote two main brands that are from multinational companies. Chudleys is fine, they sell it at our local horse food and agricultural feed depot. However I have happily used Eden, Millies Wolfheart, Guru, and the first two do small bite size for puppies. They all contain 'real' human quality food, not the meat derivatives and cereals that give huge poos.
RC really annoys me, our local little independent pet shop has now got a whole wall stacked with so-called breed specific kibble. Does a cocker really have to have a different bag from a schnauzer, westie, or poodle? It's all a marketing caper, supported by dog magazines , who make money out of their advertising space and promoted articles. Look at the smaller, British specialist companies online, who will talk to you on the phone to devise on what suits you and your pup.